Not deleting emails off server

Hello,
So I've had the htc hero for a while now and have it pulling email from my domains POP. I have the option selected to "delete mail from server" however, it doesn't. As a result at some point, the inbox on the server get past it's quota and starts bouncing emails back to folks. Why isn't the phone communicating with the server to delete the emails? Is it because I have the Mail size limit set low so if there is an attachment to download the email remains? I would not want to have every email that comes in to my account with an attachment download that picture or whatever, which could seriously fill the phones storage. If there's a work around or something I would appreciate it.
thnx